在下面幾種情況下，unless和if not不能隨便換用。
（1）當句子表示“B事物如果不停止，A事物將會發生”，我們可以用if not或者unless.
I‘ll come back tomorrow if there's not a plane strike.(ss there's a plane strike.)要不是飛機出故障了，我明天就打算回來。
Let's have dinner out-if you're not too tired.(ss you're too tired.)如果你不是太累的`話，我們出去吃飯吧。
（2）如果表示“A事物發生是因爲B事物沒有發生”，我們一般只能用if not.
I'll be glad if she doesn't come this eveing.(NOT I'll be glad unless she comes this eveing.)
如果她今天晚上沒有來，我會非常高興。
She'd be pretty if she didn't wear so much make-up.(ss she wore so much make up.)
如果她沒有化如此濃的妝，她會更漂亮。
